There have been many attempts to use photopolymerizing compositions as photosensitive image-forming layers of presensitized printing plates. Basic compositions are disclosed in a number of publications. For example, U.S. Pat. No. 3,458,311 discloses a composition comprising binder polymers, monomers and photopolymerizing initiators. U.S. Pat. No. 3,796,578 discloses composition having improved hardening efficiency obtained by introducing unsaturated double bonds into polymers to be used as binders. Compositions in which novel photopolymerization initiators are employed are disclosed in U.S. Pat. Nos. 3,549,367 and 3,751,259 and British Pat. No. 1,388,492. Only a part of these compositions are put to a practical use. However, each of these compositions is not desirable because the sensitivity is greatly influenced by the surface temperature which the resulting presensitized printing plate has at the time of imagewise exposure (which phenomenon is called temperature dependance, hereinafter).
More specifically, it has been found that changes in the sensitivity occur under ordinary plate-making conditions. In some cases the sensitivity may change by 2 to 8 times. More specifically if 10 seconds of exposure is required for producing the optimum image in a presensitized printing plate when the temperature of the plate surface is 45.degree. C., 20 to 80 seconds' exposure is necessitated when the plate is 10.degree. C. Otherwise a satisfactory image can not be formed. In view of plate-making conditions, however, a plate surface temperature of 10.degree. C. is possible when working in the early morning of winter, and it is not uncommon for the plate surface temperature to reach 45.degree. C. or higher by absorption of heat emitted from a light source when work is being carried out continuously and the printing frame is arranged at a short distance from the light source.
Under these circumstances, it is most unlikely that images of the same quality can be formed stably with the same exposure.
In addition, photopolymerizing compositions have turned out to have another disadvantage that postpolymerization caused by residual active species is observed in the photopolymerizing compositions after cessation of imagewise exposure. Accordingly, the longer the interval from imagewise exposure to development becomes, the higher the sensitivity goes up (which phenomenon is called latent image progression, hereinafter).
In practice, the sensitivities of photopolymerizing compositions may be increased by up to 2 to 8 times due to latent image progression, and this may occur with photopolymerizing compositions disclosed in the above-described patent specifications. This phenomenon becomes a serious impediment to the processing of photopolymerizing compositions into presensitized printing plates, because on a plate-making site there are distinct differences in the thickness of image lines and the tone of image patterns between when development is carried out just after imagewise exposure and when development is carried out some time after imagewise exposure.